It is well known that organic materials such as plastics, rubbers, lubricating oils, etc. are prone to oxidation and deterioration in the presence of oxygen. Oxidation of organic materials causes the loss of those intrinsic properties characteristic of the organic material. With a view to preventing deterioration, a variety of antioxidants and antiozonants have been developed; however, these stabilizers fail to prevent completely the deterioration of the desired properties of the materials to which they are added. Thus, those skilled in the art are constantly searching for new and more effective stabilizing systems which are useful for the protection of polymers and other organic materials.
Synergists have been known in the art for some time. See for example U.S. Pat. No. 3,492,336 which discloses a novel tetra-alkyl thioethyl thiodisuccinate compound for use with phenolic type antioxidants in the stabilization of polyolefins.
U.S. Pat. No. 4,254,020 discloses compounds such as 2,12-dihydroxy-4,10-dithia-7-oxatridecamethylene bis[3-(dodecylthio)propionate] as a synergist with phenolic antioxidants.
In U.S. Pat. No. 3,398,116 a thiocarboxylic acid thioether ester is used in combination with a limited group of phenolic antioxidants to stabilize poly-alphaolefins against oxidative degradation.
U.S. Pat. No. 3,758,549 discloses polyalkanol esters of alkylthio-alkanoic acids as synergists with phenolic antioxidants and U.S. Pat. Nos. 3,666,716 and 3,505,225 disclose derivatives of diphenylamine and phenylnaphthylamines as antioxidants with dialkyl 3,3'-thiodipropionates as a synergist. U.S. Pat. No. 3,450,671 discloses polyolefin compositions stabilized with dialkyl 3,3'-thiodipropionate and a polyphenol.
An article by R. Chandra, Polymer, Vol. 24, February 1983 discloses styrene-butadiene copolymers stabilized with a synergist mixture of the antioxidant 3,5-di-t-butyl-4-hydroxybenzylmercaptan in combination with 1,1,5-triphenyl-2-s-(3',5'-di-t-butyl-4'-hydroxy)benzyl-iso-4-thiobiuret bound to the polymer as a synergist.
The art of using a combination of antioxidants has been utilized by the industry, and such stabilizing systems are often highly effective. Conventionally used synergists are normally added to the material to be stabilized during a mixing operation. It was thought that for a compound to exhibit synergistic properties with an antioxidant or antiozonant it would require a certain amount of mobility within and about the polymer structure. The present invention has unexpectedly discovered that synergists can be attached or "bound" to a polymer (a segmeric unit of the polymer) and still exhibit synergistic properties with a phenolic or an amine stabilizer. Further, it has been discovered that bound synergists demonstrate activity with what is now known in the art as bound antioxidants. This is highly unexpected and contrary to numerous and various teachings in the art. It has been discovered that the particular compounds of this invention can be co-polymerized with other monomers to form a polymer that has chemically attached to it, via a covalent bond, a moiety which can synergistically enhance the antidegradative properties of phenolic and amine antioxidants.
It has also been discovered that the compounds of this invention can be grafted onto the polymeric backbone using a technique similar to that described in U.S. Pat. No. 4,155,955 which is herein incorporated by reference.
The majority of the synergistic stabilizers to date have been used in conjunction with phenolic antioxidants since use with amine antioxidants has not demonstrated synergistic properties. It was felt that synergism was simply overpowered by the excellent stabilizing properties of compounds such a N,N'-p-phenylenediamine. In this regard, it is surprising that the compounds of the instant invention demonstrate synergistic activity with amine type antioxidants. As a result of these discoveries, it has been found that the combined use of the compounds of this invention and an antioxidant brings about an unexpectedly powerful antioxidative effect. None of the cited patents or other literature in the art has disclosed or even suggests the synergist compounds disclosed in this invention.